1. Field of the Invention
The present invention relates to an engine cylinder block for a two cycle combustion engine of an air scavenging type which may be used as a drive source for a portable work machine such as a brush cutter, and also to a method for manufacturing such engine cylinder block.
2. Description of the Prior Art
It is well known that some of the two-cycle combustion engines currently available in the market employ an engine cylinder block of a type, in which a scavenging passage defining wall is provided in a region confronting the cylinder bore so as to define a part of the scavenging passage communicating between a combustion chamber and a crankcase chamber. When this type of cylinder block is formed with the use of a molding die, a scavenging port defined above the scavenging passage defining wall is in the form as undercut.
The Japanese Laid-open Patent Publication No. 58-155114, published Sep. 14, 1983, discloses formation of the scavenging port by means of, for example, a cutting process with the use of a rotary cutter after the cylinder block has been die formed. According to this Patent Document, as shown in FIG. 18 of the accompanying drawings, each of the scavenging ports 91 open at respective portions of the inner peripheral wall of the cylinder block above the associated scavenging passages 92 is cut by a rotary cutter 80, that is inclined at an angle β relative to the longitudinal axis C of the cylinder bore so as to match with the angle of inclination of the scavenging port 91 with respect of such longitudinal axis C, so as to open into the cylinder bore.
Since the rotary cutter 80 is of a cylindrical shape, the rotary cutter 80 tends to be shifted in a direction towards the longitudinal axis C of the cylinder bore in the cylinder block 90 by the effect of a contact reaction acting on the rotary cutter from the inner peripheral surface of the cylinder block 90 during the course of cutting into the inner peripheral surface of the cylinder block 90 and, therefore, the cutting to define the respective scavenging ports 91 is difficult to achieve.
Also, the resultant scavenging ports 91, when viewed from the inside of the cylinder bore in the cylinder block 90, represents a shape generally complemental to the shape of the rotary cutter 80 with its height H constant in a transverse direction P as shown in FIG. 19. Accordingly, each scavenging port 91 has an upper edge 91a extending straight in a direction perpendicular to the longitudinal axis C of the cylinder bore in the cylinder block 90 and, therefore, a portion of a piston ring mounted on a reciprocating piston movable up and down within the cylinder bore tends to get stuck on the entire width W of the upper edge 91a of the respective scavenging port 91 at one time, with a scrabbling force consequently acting on the reciprocating piston. Once this occurs, the scrabbling force acts as a resistance to a smooth movement of the reciprocating piston within the cylinder bore.
Yet, since the upper edge 91a of each of the scavenging ports 91 extends straight, the scavenging ports are fully opened over the entire width W thereof from the very beginning of the scavenging stroke, accompanied by a rapid increase of the opening area of the scavenging ports to result in an occurrence of an undesirable blow-by of the scavenging gas from an exhaust port 93.